1. Technical Field
Some technical definitions relevant to the disclosure include “non-spore forming bacteria” which is a known term used for pathogenic and spoilage bacteria that cannot form bacterial spores and can be destroyed or controlled by a heat treatment, refrigerated anaerobic storage, antibacterial substances and other methods known in the art used alone or in combination. Another relevant term is “spore forming bacteria”, which includes pathogenic and spoilage bacterial capable of forming very resistant structures called bacterial spores (also termed endospores) that are not necessarily destroyed or controlled by the common methods known in the art for the control of non-spore forming bacteria and require specific treatments for their inhibition and/or inactivation. Additionally, both types of bacteria can exist in nature in a “vegetative state” also termed viable cells; however spore-forming bacteria can also exist in a “spore-state” which is more resistant to chemical and physical treatments for their inactivation. In the field of food technologies there are additional bacterial states for spore forming bacteria that are artificially created by the application of heat termed “heat-shocked spores” and/or pressure “pressure-shocked spores”. The artificial states generated in the food industry result in an even higher resistance of the spores to their inactivation by chemical and physical means and in some food systems need to be controlled in order to inhibit their germination into the vegetative form of the spore forming bacteria and subsequent spoilage of the food and/or toxin production.
Some additional technical definitions relevant to the disclosure include “antimicrobial” which is a term used to describe an agent able of inhibiting the growth of a wide class of microorganisms including bacterias, fungus, molds, viruses or yeast. Whereas “antibacterial” is a term used to describe an agent able of inhibiting the growth of spore forming or non-spore forming bacterias in a vegetative state. And the term “spore germination inhibiting activity” or “spore germination inhibiting effect” refers to spores from spore forming bacteria, except for where otherwise indicated. Additionally “raw extract” is a term used to define an extract obtained by mixing Persea spp. (avocado) tissue with a non-polar or polar solvent and that contains a broad spectrum of chemical compounds other than acetogenins with antimicrobial, antibacterial and spore germination inhibiting effect. Whereas “extract enriched in acetogenins” is the term used to define an extract obtained after the removal of compounds different from acetogenins with antimicrobial, antibacterial and spore germination inhibiting effect.
This disclosure relates to the food and pharmaceutical arts. In particular it relates to a method of inhibiting vegetative cells, spore germination and growth of gram positive bacteria by the use of chemical compounds naturally present in Persea spp.
The disclosure also relates to the medical arts. In particular it relates to a method of inhibiting the growth of pathogenic spore forming bacteria in the body including the gastrointestinal tract of a human or non-human vertebrate by the use of an antimicrobial extract with specificity for this type of bacteria.
It is known in the discipline of food processing that food products with pH values>4.6 (commonly known in the food industry as low-acid foods) can experience the germination and growth of spore forming bacteria. Of particular interest for the food industry is the use of food additives capable of inhibiting spore germination and vegetative cell growth from pathogenic spore forming microorganisms such as Clostridium botulinum, Clostridium perfringens and Bacillus cereus, among others. Under the proper food environments such as enclosed containers or anaerobic conditions generated within the food matrix the spores from these pathogenic microorganisms can germinate and grow to harmful numbers of bacterial cells and in some cases can produce toxins jeopardizing human health. Particularly, the proteolytic and non-proteolytic strains of Clostridium botulinum are a major concern for the food industry because of the potential germination of their bacterial spores in foods and the production of potent neurotoxins. Nitrites are the most commonly used food additives in the food industry to retard/inhibit the growth of spore forming pathogenic bacteria in refrigerated low-acid foods. However, there is a consumer and industrial long standing interest to reduce the utilization of synthetic food additives, particularly nitrite compounds. Other food additives that have been used for the same purposes include nisin (Rayman, 1981), recombinant peptides (Tang et al., 2008), 5-aminosalicylates (Lin and Pimentel, 2001) and ethyl lauroyl arginate (Beltran et al., 2011). Additionally, there have been prior patents and articles related to antimicrobial compounds from natural origin that act against bacterial vegetative cells. Many natural sources have been reported to contain antimicrobial compounds mainly lipophilic, although some hydrophilic compounds have also shown activity. Reports of antimicrobial compounds of this nature are available in literature.
The disclosure also relates to an important public health concern that is the ability of pathogenic species, especially the gram positive Listeria monocytogenes, to grow at commercial refrigeration temperatures at which processed foods are normally stored before final consumption. Listeria monocytogenes is a non-spore forming pathogenic bacteria of special concern for ready-to-eat meats and dairy products; as such foods are frequently not heated by the user prior to consumption. Consumption of foods contaminated with Listeria monocytogenes are known in the art to increase the risk of infection, especially among infants, the elderly, pregnant women, and any immune compromised individuals.
For the purposes of this disclosure a sporocidal agent is a substance with the ability to kill at least some types of bacterial spores whereas a sporostatic agent is a substance that has the ability to inhibit the growth and reproduction of at least some types of bacterial spores. Spore germination inhibitors include both sporicidal and sporostatic agents.
In addition, except for where otherwise indicated, depictions of the compounds described below are intended to encompass all stereoisomeric forms thereof which includes (R) and (S) forms and cis (Z) and trans (E) forms of the compounds. For the purposes of this disclosure, the trans (E) form can include a terminal alkene which has the formula —CH═CH2 (see e.g. (2R,16E)-1-acetoxy-2-hydroxy-4-oxo-nonadeca-16,18-diene below).
2. Description of the Related Art
Jensen in 1951 (U.S. Pat. No. 2,550,254) obtained an acetone extract from avocado (Persea gratissima) seed having antibacterial activity against vegetative cells from Staphylococcus aureus, Bacillus subtilis, Aspergillus glaucus, Penicillium notatum, and Achromobacter perolens. This extract was found to be inactive against Esherichia coli, Pseudomonas fluorescens and Penicilliun camemberti. The same author in 1953 (Canada Patent 494,110) refers to avocado (Persea americana) seed as another natural source that might be used to obtain an extract with antimicrobial activity. Valeri and Gimeno (1954) extracted avocado seeds with petroleum ether and reported that the resulting crude wax inhibited growth of Micrococcus pyogenes and Sarcina lutea, but not growth of B. subtilis or of E. coli. The prior art indicates that avocado seeds contain antimicrobial compounds but the specific bioactivity of the extract against particular microorganisms clearly depends on the method of extraction, which in the end impacts the chemical composition of the extract.
In the related art, some compounds have been isolated from avocado seed extracts and tested to inhibit the growth of certain microorganisms (bacteria, yeasts and fungi). Kashman et al. (1969) isolated and elucidated the structure of eight compounds from a hexane extract of avocado fruit and seeds and a number of derivates thereof were prepared, obtaining higher yields from the seeds than the fruit. All compounds showed by Kashman (1969) belong to the same group of long chain aliphatic compounds, with one end being unsaturated and the other end highly oxygenated. Interestingly the compounds were divided by the authors in pairs differing only by having a double or triple bond at the end of the chain. The isolation of these compounds was with the aim of performing a chemical characterization and not for obtaining bioactive components (not bioactivity-guided isolation). Additional studies were then performed to evaluate their antimicrobial activity against Bacillus subtilis, Bacillus cereus, Salmonella typhi, Shigella dysenteriae, Staphylococcus aureus, Candida albicans, Saccharomyces cerevisiae (ATCC 7752 and S 288C) (Néeman et al. 1970). Only six of twelve long-chain aliphatic compounds tested demonstrated inhibitory effects against some of the microorganisms but only 1, 2, 4-trihydroxy-n-hepadeca-16-en was capable of inhibiting the growth of all the microorganisms included in their study in a disc inhibition antimicrobial test that used 0.05 mg of the compound. The authors concluded that when the hydroxyl groups on the oxidized part of the compound were totally, or partially, acetylated, the antibacterial activity was greatly weakened (Néeman et al. 1970). Therefore acetogenins, which are the acetylated form of the above mentioned long chain aliphatic compounds, did not inhibit the growth of the previously mentioned microorganisms. Baratta et al. (1998) more recently conducted a study to evaluate the antimicrobial and antioxidant properties of an extract of essential oils from plants including laurel (Laurus nobilis) form the Lauraceae family but did not include the genus Persea. 
Recently, Ugbogu and Akukwe (2009) reported on the antimicrobial effects of seed oils from Persea gratissima Gaerth F, among other plant seed oils, against clinical isolates of non-spore forming bacteria that included Escherichia coli, Proteus mirabilis, Pseudomonas aeruginosa, Staphylococcus aureus and Staphylococcus epidermis. The authors reported potential use of Persea seed oils in the treatment of wounds. Chia and Dykes (2010) also prepared ethanolic extracts from the epicarp and seed of Persea Americana Mill. vars. Hass, Shepard and Fuerte. They reported that at concentrations between 104.2-416.7 μg/ml, the extract showed antimicrobial activities against the growth of vegetative cells of both gram positive and gram negative bacteria; the authors also prepared a water extract that only inhibited the growth of Listeria monocytogenes (93.8-375 μg/ml) and Staphylococcus epidermis (354.2 μg/ml). Activity against Clostridium or Bacillus genus was not evaluated for the ethanolic or aqueous extract. Rodriguez-Carpena et al. (2011), in an attempt to isolate molecules with antibacterial activities, prepared extracts from the peel, pulp, and seed of two avocado cultivars (Persea Americana Mill.) using three different solvents that included ethyl acetate, acetone (70%) and methanol (70%). The authors tested the antibacterial properties of the extracts against a panel of vegetative cells from non-spore forming and spore forming bacteria, concluding that their antibacterial activity was moderate and it was attributed to the presence of phenolic compounds in their extracts. Therefore the prior cited studies did not successfully performed the isolation or chemical identification of the components potentially responsible for the observed bioactivities or tested bacterial spores, heat-shocked spores or pressure-shocked spores.
Similarly, other authors have tested the antimicrobial properties of the avocado plant, against microorganisms other than bacteria. Prusky et al. (1982) described the presence of 1-acetoxy-2-hydroxy-4-oxo-heneicosa-12,15-diene (Persin) in the peel of unripe avocado fruits and attributed to the molecule the antimicrobial activity against Colletotrichum gloeosporioides, a fungus that causes anthracnose, a known problem encountered during storage of avocado fruits. The compound was isolated by Thin Layer Chromatography from an ethanolic extract partitioned with dichloromethane. This compound was later termed “persin” (Oelrichs et al., 1995), and was confirmed by other authors as the constituent of avocado with the highest inhibitory activity against the vegetative growth of the fungi Colletotrichum gloeosporioides tested in vitro (Sivanathan and Adikaram, 1989; Domergue et al., 2000), and with the capability to inhibit its fungi spore germination and germ tube elongation (Prusky et al., 1991a). Persin inhibited fungi spore germination completely at 790 μg/ml and the concentration of this compound in the peels was greatly reduced during ripening (Prusky et al., 1982). A monoene with similar structure, 1-acetoxy-2,4-dihydroxy-n-heptadeca-16-ene, also demonstrated bioactivity against Colletotrichum gloeosporioides but it was 3 fold lower than that of persin. Interestingly, a 1:1 mixture of both antifungal compounds showed synergistic activity and increased the percent of inhibited germ tube elongation of germinated conidia (Prusky et al., 1991b). Other compounds such as 1-acetoxy-2-hydroxy-4-oxo-heneicosa-5,12,15-triene (Domergue et al., 2000) have also been proven to have antifungal bioactivity. This last compound has been termed “Persenone A” (Kim et al., 2000a), however none of the isolations has been performed based on its bioactivity or with the aim of discovering novel compounds or mixtures with increased bioactivity. Most of the prior art publications have focused on finding molecules to prevent postharvest damage.
Additional bioactivities that have been reported for acetogenins included insecticidal, antitumoral, and antihelmintic properties. Persin has shown to have insecticidal activity, inhibiting the larval feeding of silkworm larvae Bombyx mori L., at a concentration in the artificial diet of 200 μg/g or higher (Chang et al., 1975; Murakoshi et al., 1976). More recently, Rodriguez-Saona et al. (1997) demonstrated the effects of persin on Spodoptera exigua, a generalist feeder insect, that does not feed on avocados, but is one of the major pests of many vegetables. Inhibitory effects were observed for both larval growth and feeding at concentrations of 200 μg/g and 400 μg/g of diet, respectively.
Persin was also identified as the active principle present in avocado leaves that induces lactating mammary gland necrosis of mice at a dose rate of 60-100 mg/kg, at doses above 100 mg/kg necrosis of mice myocardial fibers may occur, and hydrothorax may be present in severely affected animals (Oelrichs et al., 1995). Derived from this effect, this compound and others obtained from avocado leaves were patented as treatment for ovarian and breast cancer in mammals (Seawright et al., 2000). The compounds were administered orally up to 100 mg/kg of body weight of mammal being treated, but preferably on a number of consecutive days at a concentration of 20-40 mg/kg of body weight to avoid the previously reported toxic effects. As it was previously noted, the concentration of these compounds in the avocado pulp is greatly reduced during ripening to values lower than 1500 μg/g (Kobiler et al, 1993); therefore more than 0.8 kg of avocado pulp should be consumed daily by a 60 kg human to reach the anticancer effect and even a higher concentration to reach the cytotoxic effects. The annual therapeutic dose proposed for cancer treatment is 160-fold higher than the actual annual per capita consumption of avocado in the United States (1.8 kg or 4.1 pounds) reported by Pollack et al (2010).
Persenone A, and its analog 1-acetoxy-2-hydroxy-5-nonadecen-4-one (Persenone B), along with Persin were found to inhibit superoxide (O2−) and nitric oxide (NO) generation in cell culture, activities that were associated by the authors to therapeutic uses as cancer chemopreventive agents in inflammation-related organs (Kim et al., 2000a, 2000b and 2000c). In vitro results demonstrated that they have equal or better activity than DHA (docosahexaenoic acid), a natural NO generation inhibitor. The IC50 values were in the range of 1.2-3.5 μM for acetogenins and 4.5 μM for DHA (Kim et al., 2000a). Additionally, 1-acetoxy-2,4-dihydroxy-n-heptadeca-16-ene, persin and persenone A showed inhibition of acetyl CoA carboxylase (ACC) activity, in the IC50 value range 4.0-9.4 μM (Hashimura, 2001). Authors concluded that since ACC is involved in fatty acids biosynthesis, those compounds have a potential use as fat accumulation suppressors to avoid obesity.
Most of the extraction methods for long-chain fatty acid derivatives require a previous step to recover the oil or the use of organic solvents such as hexane. The method of extraction for the identified antimicrobial compounds used by Kashman, Neeman and Lifshitz, (1969) used hexane at boiling temperatures. Broutin et al. in 2003 (U.S. Pat. No. 6,582,688 B1) developed a method for obtaining an extract from avocado fruit oil enriched in certain class of long chain aliphatic compounds, such as furan lipid compounds and polyhydroxylated fatty alcohols. The authors claimed that different compositions of those non polar compounds may be used in different therapeutic, cosmetic and food applications. However the chemical composition of the extract obtained by their process or the content of the active molecule(s) was not specified for its use as an antimicrobial agent. Considering the toxicity of some of the compounds that might be present in a raw extract, it is extremely important to define the minimal concentration required to attain the desired effects (see U.S. Patent Application Publications 2006-0099323 and 2009-0163590).
Even if some acetogenins have been proven to have antimicrobial activity against vegetative cells of bacteria, the preliminary art does not show any reports on the bio-assay guided isolation of the antimicrobial compounds from avocado (Persea americana) against microorganisms, particularly sporulated forms. The present disclosure provides a series of steps for a process to obtain isolated compounds and/or a composition that concentrates the naturally occurring antibacterial compounds in Persea americana that inhibit the growth of vegetative and sporulated states of spore forming bacteria. The isolation of compounds based on inhibition of sporulated microorganisms do not form part of the teaching of the prior art. More importantly, the synergistic effect of the specific compounds in partially purified mixtures is also part of the present disclosure. The inventors found intriguing that the partially purified extracts and/or mixtures of isolated compounds possess spore germination inhibiting properties, such as sporostatic and/or sporocidal properties, and in some instances even better effects than the isolated compounds alone. The chemical identity and specificity of the active compounds against spore forming microorganisms has never been previously reported nor the heat or pressure stabilities of the bioactive compounds under commercially applicable processing conditions.
Maseko (2006) proposed a simple method to produce a non acetylated fatty acid derivative called (2R,4R)-1,2,4-trihydroxyheptadeca-16-ene by using (S)-malic acid as a cheap source of the triol fragment and the Grignard reaction to achieve the elongation of the aliphatic chain. This precursor could be used for the synthesis of most acetogenins in avocado oil. This molecule was produced as an analytical standard in Masenko (2006) and in prior art Néeman et al. (1970) had shown the potential of the compound as an antimicrobial agent against Staphylococcus spp., a non-spore forming bacteria. None of the cited authors tested any specific antimicrobial properties against spore forming bacteria nor a method to produce acetogenins with this particular effect.
In reference to the prior art on antimicrobial substances to be used for the specific control of vegetative cells of Listeria monocytogenes in refrigerated foods, U.S. Pat. No. 5,217,950 suggested the use of nisin compositions as bactericides for gram positive bacteria. U.S. Pat. Nos. 5,573,797, 5,593,800 and 5,573,801 disclose antibacterial compositions which include a combination of a Streptococcus or Pediococcus derived bacteriocin or synthetic equivalent antibacterial agent in combination with a chelating agent. U.S. Pat. No. 5,458,876 suggests the combination of an antibiotic (such as nisin) with lysozyme as an antibacterial. In this case, lysozyme breaks down the cell wall and weakens the structural integrity of the target cell so that the antibacterial agent becomes more effective in damaging or killing the bacterial cell. In particular, this combination proves to be effective in improving the antibacterial—efficacy of nisin against Listeria monocytogenes, yielding a significant reduction, though not a complete elimination, of Listeria at safe and suitable levels of use. U.S. Pat. No. 6,620,446B2, describes an antibacterial composition for control of gram positive bacteria in food applications that may be used as an ingredient or applied to a food surface. This composition includes nisin, and/or lysozyme and beta hops acids in order to reduce or eliminate gram positive spoilage or pathogenic bacteria, and, most especially, all strains of the harmful pathogen Listeria monocytogenes. Perumalla and Hettiarachchy (2011) reported that green tea extract and grape seed extract (polyphenolic and proanthocyanidin rich compounds) had antimicrobial activities against major food borne pathogens like Listeria monocytogenes, Salmonella typhimurium, Escherichia coli O157:H7, and Campylobacter jejuni. Furthermore, they have demonstrated synergism in antimicrobial activity when used in combination with organic acids (malic, tartaric acid, benzoic acids etc.), bacteriocins like nisin or chelating agents like EDTA in various model systems including fresh products (fruits and vegetables), raw and ready-to-eat meat and poultry products.
Given the difficulties associated with obtaining extracts with adequate antibacterial, antimicrobial or spore germination inhibiting activities, the development of resistance by bacteria, microbes and spores to known antibacterial, antimicrobial, spore germination inhibiting compounds and compositions, and the desire for food products and medicaments of natural origin, there still exists a need in the art for additional antibacterial, antimicrobial or sporicidal compounds and compositions preferably obtained from economically feasible sources such as plant processing by-products and waste.